1. Field of the Invention
This invention relates to novel Tl--Pb--Ca--Sr--Cu--O compositions which are superconducting.
2. References
Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconducting phase in the La--Ba--Cu--O system with a superconducting transition temperature of about 35 K. This disclosure was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science 235, 567 (1987), Chu et al., Phys. Rev. Lett. 58, 405 (1987), Cava et al., Phys. Rev. Lett. 58, 408 (1987), Bednorz et al., Europhys. Lett. 3, 379 (1987)]. The superconducting phase has been identified as the composition La.sub.1-x (Ba,Sr,Ca).sub.x CuO.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett 58, 908 (1987), disclose a superconducting phase in the Y--Ba--Cu--O system with a superconducting transition temperature of about 90 K. Cava et al., Phys. Rev. Lett. 58, 1676 (1987), have identified this superconducting Y--Ba--Cu--O phase to be orthorhombic, distorted, oxygen-deficient perovskite YBa.sub.2 Cu.sub.3 O.sub.9-.delta. where .delta. is about 2.1 and present the powder x-ray diffraction pattern and lattice parameters.
C. Michel et al., Z. Phys. B--Condensed Matter 68, 421 (1987), disclose a novel family of superconducting oxides in the Bi--Sr--Cu--O system with composition close to Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. A pure phase was isolated for the composition Bi.sub.2 Sr.sub.2 Cu.sub.2 O.sub.7+.delta.. The X-ray diffraction pattern for this material exhibits some similarity with that of perovskite and the electron diffraction pattern shows the perovskite subcell with the orthorhombic cell parameters of a=5.32 A (0.532 nm), b=26.6 A (2.66 nm) and c=48.8 A (4.88 nm). The material made from ultrapure oxides has a superconducting transition with a midpoint of 22 K. as determined from resistivity measurements and zero resistance below 14 K. The material made from commercial grade oxides has a superconducting transition with a midpoint of 7 K.
H. Maeda et al., Jpn. J. Appl. Phys. 27, L209 (1988), disclose a superconducting oxide in the Bi--Sr--Ca--Cu--O system with the composition near BiSrCaCu.sub.2 Ox and a superconducting transition temperature of about 105 K.
The commonly assigned application, "Superconducting Metal Oxide Compositions and Process For Making Them", Ser. No. 153,107, filed Feb. 8, 1988, a continuation-in-part of Ser. No. 152,186, filed Feb. 4, 1988, disclose superconducting compositions having the nominal formula Bi.sub.a Sr.sub.b Ca.sub.c Cu.sub.3 O.sub.x wherein a is from about 1 to about 3, b is from about 3/8 to about 4, c is from about 3/16 to about 2 and x=(1.5 a+b+c+y) where y is from about 2 to about 5, with the proviso that b+c is from about 3/2 to about 5, said compositions having superconducting transition temperatures of about 70 K. or higher. It also discloses the superconducting metal oxide phase having the formula Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w wherein z is from about 0.1 to about 0.9, preferably 0.4 to 0.8 and w is greater than zero but less than about 1. M. A. Subramanian et al., Science 239, 1015 (1988) also disclose the Bi.sub.2 Sr.sub.3-z Ca.sub.z Cu.sub.2 O.sub.8+w superconductor.
Y. Yumada et al., Jpn. J. Appl. Phys. 27, L996 (1988), disclose the substitution of Pb for Bi in the series Bi.sub.1-x Pb.sub.x SrCaCu.sub.2 O.sub.y where x=0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0. The T.sub.c increases from 75.5 K. for x=0, no Pb present, to a maximum of 85.5 K. for x=0.5. T.sub.c decreases for higher Pb content to 76 K. for x=0.7. No superconductivity was observed for the samples with x=0.9 and x=1.
M. Takano et al., Jpn. J. Appl. Phys. 27, L1041 (1988), disclose that partial substitution of Pb for Bi in the Bi--Sr--Ca--Cu--O system results in an increase in the volume fraction of the high T.sub.c phase. Coprecipitated oxalates containing the relevant ions in various ratios underwent thermal decomposition below 773 K. The samples in powder form were then heated in air to 1073 K. for 12 hours and, after being formed into pellets, at 1118 K. for various periods which extended to more than 240 hours in some cases. A starting composition of Bi:Pb:Sr:Ca:Cu=0.7:0.3:1:1:1:8 was heated at 1118 K. for 244 hours. The high-T.sub.c phase shows an onset of superconductivity at around 115 K. This phase forms plate-like crystals and analysis of these crystals indicates that the cationic ratio is Bi:Pb:Sr:Ca:Cu=67:5:100:85:180 so that there is considerably less Pb in the high-T.sub.c than in the starting material.
M. Mizuno et al., Jpn. J. Appl. Phys. 27, L1225 (1988), also disclose that the addition of Pb to the Bi--Sr--Ca--Cu--O system results in an increase in the volume fraction of the high-T.sub.c phase and a lowering of the optimum temperature to obtain this phase to about 855.degree. C.
E. V. Sampathkumaran et al., J. Phys. F: Met. Phys. 18, L163 (1988) disclose that the partial substitution of K or Pb for Bi in the Bi.sub.4 Ca.sub.3 Sr.sub.3 Cu.sub.4 O.sub.4 results in an enhancement of the fraction of the phase superconducting at about 110 K.
Z. Z. Sheng et al., Nature 332, 55 (1988) disclose superconductivity in the Tl--Ba--Cu--O system in samples which have nominal compositions Tl.sub.2 Ba.sub.2 Cu.sub.3 O.sub.8+x and TlBaCu.sub.3 O.sub.5.5+x. Both samples are reported to have onset temperatures above 90 K. and zero resistance at 81 K. The samples were prepared by mixing and grinding appropriate amounts of BaCO.sub.3 and CuO with an agate mortar and pestle. This mixture was heated in air at 925.degree. C. for more than 24 hours with several intermediate grindings to obtain a uniform black oxide Ba--Cu oxide powder which was mixed with an appropriate amount of Tl.sub.2 O.sub.3, completely ground and pressed into a pellet with a diameter of 7 mm and a thickness of 1-2 mm. The pellet was then put into a tube furnace which had been heated to 880.degree.-910.degree. C. and was heated for 2-5 minutes in flowing oxygen. As soon as it had slightly melted, the sample was taken from the furnace and quenched in air to room temperature. It was noted by visual inspection that Tl.sub.2 O.sub.3 had partially volatilized as black smoke, part had become a light yellow liquid, and part had reacted with Ba--Cu oxide forming a black, partially melted, porous material.
Z. Z. Sheng et al., Nature 332, 138 (1988) disclose superconductivity in the Tl--Ca--Ba--Cu--O system in samples which have nominal compositions Tl.sub.2 Ca.sub.2 BaCu.sub.3 O.sub.9+x.
R. M. Hazen et al., Phys. Rev. Lett. 60, 1657 (1988), disclose two superconducting phases in the Tl--Ba--Ca--Cu--O system, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 and Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8.